Communications system

ABSTRACT

A multi-point communications system joins a series of communications units in a series chain while providing communications between and among all of the units. Each communication unit comprises a pair of frames, preferably operating under E 1 /T 1  protocols connected in a back-to-back arrangement through the framers&#39; local serial busses. Communication data is placed on and extracted from a designated set of channels on the serial busses while preamble data, controlling the passage and routing of the communications data, is placed on a separate set of bus channels. One of the interconnected communications units is designated as the “parent”, and provides supervisory control over communications data, timing and flow. Designation of the parent unit may be performed on a dynamic basis to allow the communications system to dynamically adapt to changes in system size as communications units are added or dropped.

[0001] The present invention relates to a multi-station communicationsystem, and in particular to a communications system which canreconfigure itself to size changes and which provides a communicationsprotocol which allows the use of point-to point communication to enablea multi-station system.

BACKGROUND OF THE INVENTION

[0002] The use of communications systems for the transfer of voice anddata between and among locations has become widespread. The use ofindustry-wide standards and protocols has accelerated such use, as theyallow the designers of communication systems in accordance with suchstandards to take comfort in knowing in advance that a particular levelof performance can be realized. In addition, the use of components andsystems utilizing such standards allows the specification of componentsand sub-systems to be simplified, since compliance with a particularstandard eliminates the need for tailoring the system to accommodate aparticular element's thereof characteristics, and can permit orfacilitate the interchange of components from various sources withoutcompatibility concerns.

[0003] Typical communication systems utilize timing and synchronizationsignals to control event timing and to allow the passage of data betweenthe units in a coordinated manner. One of the interconnected units istypically designated as a master, with the remaining units designated asslaves. The master unit generates the timing signals which arecoordinated, or slaved, to by the remaining units. In this manner aunitary, system-wide time base and synchronization protocol isestablished and maintained.

[0004] It is of course important that coordination and synchronizationis maintained as the system grows or shrinks. In many applications thereconfiguration of a system is done by a skilled technician, having theability to reconfigure the system as appropriate. Further, other systemsallow the addition and deletion of slave units with automaticreconfiguration, improving the versatility of such systems andsimplifying the reconfiguration process.

[0005] Time Division Multiplexing (TDM) is a particularly popular andwell accepted transmission method for telephony systems. Some TDMstandard protocols are known as ISDN/E1/T1. They provide a robustarchitecture but are most commonly used in rack-mounted systems or thelike for point-to-point communications in which interconnectionsbetween, and communications among, more than two users or locations isnot typically contemplated. In an E1/T1 type system, one of theinterconnected units must be configured to provide the necessary timingand sync signals for the unit or units which follow. In addition, E1/T1type systems cannot typically be used to enable selective communicationsbetween a plurality of communications units as may be required in a railcar communications system and the like.

[0006] Rail cars have adopted the use of communication systems utilizingmicroprocessors to control operational functions and passengercommunications. Towards such ends, the communications units in each carmust be interconnected. Such rail car communications systems, however,present system interconnection requirements which differ from many othersystems, and particular do not typically permit E1/T1 protocol systemsto be employed. In addition to a rail car system requiring a varyingnumber of communications units as a train is assembled from individualcars, a particular car may be attached to an existing train or sequenceof cars at either end thereof. In a similar manner, individual cars canbe removed from a train and/or their location within a train shuffled.This dynamic reconfiguration places additional burdens on thecommunication system between the cars. The coupling and uncoupling ofrail cars is typically accomplished by personnel not having thenecessary technical training required to reconfigure the communicationsystem to accommodate the new car, nor during normal train operations dothey typically have the time or are they often even permitted access tothe communications system. Even communications systems which have thecapacity to automatically accept the addition or deletion of additional“slave” units are ill-equipped to accommodate the loss of a designatedmaster unit.

[0007] It is accordingly a purpose of the present invention to provide amultiple unit communications system or network incorporating E1/T1 typearchitecture.

[0008] Another purpose of the present invention is to provide acommunications system, such as a communications system utilizing theE1/T1 protocol, which allows a plurality of individual communicationsunits to be interconnected and operated in a flexible manner, andwithout the need for manual reconfiguration.

[0009] Yet another purpose of the present invention is to provide amethod and apparatus for the automatic mediation and assignment of aresponsibility, such as timing and/or synchronization of communicationsunits, between peer units in a multiple unit system, and particularly acommunication system.

[0010] A further purpose of the present invention is to provide a methodand apparatus for dynamically allocating master and non-masterrelationships among E1/T1 protocol communications units as they arejoined to and separated from each other within a communications system.

[0011] Still another purpose of the present invention is to provide amulti-drop communications system utilizing E1/T1 type architecture inwhich communications between units may be controlled by a parent unit,wherein communications are passed from or to the parent and a pluralityof child units.

BRIEF DESCRIPTION OF THE INVENTION

[0012] In accordance with the foregoing and other objects and purposes,a first aspect of the present invention provides a method and apparatusin which communications units comprising a pair of E1/T1 type protocolcommunication devices or framers can be interconnected to form amultiple unit series communications system or network capable ofsupporting communications among or between any of the communicationsunits in the system. While each of the E1/T1 framers operates in apoint-to-point manner, the overall system architecture and configurationallows communications to be established and maintained between and amongindividual communications units, each one of which may be associatedwith a different rail car.

[0013] A second aspect of the invention provides a method and apparatusfor dynamically configuring an electrical system, such as acommunications system, having a plurality of interconnected units, toallow assignation of one of the units to perform a task from a pluralityof the interconnected units capable of performing the task, and then toconfigure the system in response to the assignation and maintain thesystem in an operational mode, dynamically reconfiguring the system,including re-assignation of the task as required, and particularly asunits are added or dropped from the system.

[0014] In further accordance with the first aspect of the invention,each communications unit of the communications system comprises a pairof E1/T1 protocol framer units, coupled together in a “back-to-back”orientation whereby the two framers form first and second communication“ends” which are enabled to permit point-to-point communications betweenthe communications unit and adjacent communications units to which thecommunications unit is directly connected. Means are provided in eachcommunications unit, in association with the coupling of the framers toeach other within the communications unit, for controlling anddispatching data which is locally generated for transmission by theframers of the communications unit and for redirecting from one framerto the other data received by either of the framers of thecommunications unit.

[0015] Communication is established between communications units on apoint-to-point basis, but with each communication being passed along theentire length of the system. To mediate the passage of communicationsand other data along the path of interconnected communications units,one of the communications units is designated as a “parent” unit, theremaining units of the interconnected system designated as “children.”Preferably, communications are directed by the parent communicationsunit to occur between the designated parent unit and a particular one ormore child units. A child is capable of effecting a transmission only inresponse to permission granted by the parent. Each transmission,however, may be passed along the entire length of the communicationssystem to allow all units to receive transmitted data for processing andaction.

[0016] In order to allow interconnected units of an electrical componentsystem, such as a communications system, to configure itself anddynamically reconfigure itself as units are added or dropped, and inaccordance with the second aspect of the invention, the electricalcomponent units capable of or initially designated as having thepotential for performing a particular task, such as the generation oftiming and clock signals for a communications system, are identified andeach is assigned an individual identifier. The identifiers are thenpassed to the other identified units and compared. If any two of theidentified units have the same identifier, all the identified units arereassigned new identifiers. The comparison is repeated, and newidentifiers assigned, until each identified unit has a uniqueidentifier. One of the unique identifiers is then chosen, preferably ona random basis, to designate the unit to perform the task and the unitis so enabled. The remaining units of the system are then configured tointerface with the enabled unit. The procedure can be performed on anongoing basis to accommodate dynamic changes to the system.

[0017] The invention may be embodied in a communications systemcomprising a plurality of communications units coupled together in aseries relationship with two opposed end or terminal units and a varyingnumber of intermediate units therebetween. In such a configuration theinvention provides for the continued dynamic assignment of one of theterminal units as both a parent unit and as the source for timingsignals for the communications system, and for the continued dynamicconfiguration of the remaining coupled communications units inassociation therewith as child units, whereby changes to the length ofthe communications system are immediately compensated for. The inventionis thus of particular applicability to communications systems operatingunder E1/T1 and similar protocols which require the designation of aparticular communications unit to be the source of timing signals forthe system.

[0018] Each communications unit is individually characterized either aterminal unit or an intermediate unit. This may be accomplished byconsideration of signals normally generated by the communications unitbased upon its connection status with other units. Each terminal unit isthen assigned an identifier, which is passed along the communicationssystem to the other terminal unit. The received identifier is comparedto the recipient terminal unit's identifier. The identifiers are varieduntil the comparison process verifies that the two terminal units havedifferent, and thus unique, identifiers. Once this is accomplished oneof the identifiers is designated, and the terminal unit having thatidentifier is configured appropriately to generate the necessary systemsignals. The remaining communications units are then configured inaccordance with their status as either an intermediate unit or as theremaining terminal unit.

[0019] In connection with communications systems operating under anE1/T1 protocol, unit status can be determined by consideration of async/loss-of-sync status signals normally generated by the individualframer units. The identification code may by a one-bit long data element(on/off), generated, assigned and reassigned randomly. The combinationof the sync/loss-of-sync and identifier signals associated with eachcommunications unit can be multiplexed together to form a data wordwhich is processed to generate the required configuration instructionsfor the individual communications units.

[0020] In a preferred embodiment, where the terminal communications unitgenerating the system-wide timing and sync signals is designated as the“parent” unit for the system and all remaining units being designated asequal “children”, communications can be effected between units by theparent polling each of the children for permission to send data. Theparent allows child units, on a one-by-one basis, to transmit data tothe parent which is also passed throughout the system and received byall children to be acted upon or processed as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] A fuller understanding of the invention will be accomplished uponconsideration of the following explanation of a preferred, butnonetheless illustration embodiment thereof taken in association withthe annexed drawings, wherein:

[0022]FIG. 1 is a block diagram of a portion of a communications systemin accordance with the invention;

[0023]FIG. 2 is a block diagram of individual communications unitscoupled together into a four-unit communications system, depicting theframers therein and illustrating the interconnection of the framers andan adaptive mode logic control circuit therein for dynamicallyconfiguring the framers of the communications system in accordance withthe invention;

[0024]FIG. 3 is a block diagram of the two left-most pair ofcommunications units of FIG. 2, further illustrating how thecommunications units are dynamically configured in accordance with theinvention,

[0025]FIG. 3A is a logic chart for the configuration signals generatedby the invention;

[0026]FIG. 4 is a detailed block diagram detailing the datacommunications configuration of an individual communications unit;

[0027]FIG. 5 is a block diagram depicting the adaptive mode logiccontrol circuit of the invention which generates the configurationsignals;

[0028]FIG. 6 is a state control diagram for a communications unitoperating in accordance with the invention;

[0029]FIG. 7 is a schematic diagram of a logic processor for a statecontroller circuit portion of the adaptive mode circuit of FIG. 5; and

[0030]FIG. 7A is a chart of the inputs to the logic processor.

DETAILED DESCRIPTION OF THE INVENTION

[0031] With initial reference to FIG. 1, the present invention isdirected to enabling the configuration of a plurality of interconnectedelectrical units, such as communications units 10-14, each of whichdefines a communications node. Each of the communications units 10-14may be located, for example, in a different rail car, the rail carsbeing coupled together to form a train whereby the communications unitsare correspondingly coupled together through multiple channel busses 18running between the rail cars. Each of the communications units mayoperate under an E1/T1 protocol, and thus is in point-to-pointcommunications with its immediately adjacent node(s). One or more audioand/or data devices 20 may be associated with each communications unit,and generates and/or broadcasts or displays the audio and other datawhich are transmitted by the communications system between and amongnodes as required. Control circuitry 22 in each communications unitallows the data to delivered along the entire communications system,despite the point-to-point configuration of the individual framersemployed in each communications unit. Utilizing the architecture of thepresent invention, an unlimited umber of nodes made be coupled togetherin a series chain with communications enabled along the entire chain asindividual communications units or nodes are added or dropped.

[0032] Referring next to FIG. 2, each of the nodes or communicationsunits 10-16 includes a pair of E1/T1 framers, such as 24, 26 in end orterminal unit 10; framers 24′, 26′ in intermediate communications unit12 or 14, or framers 24″, 26″ in second end unit 16. Each framer is aunit as known in the art, providing interconnects for a bus channel 18to connect with another remote framer for long distance communications.Each of the channels can comprise a large number, typically upwards of30, individual data busses. Each individual bus may carry a differentform of data, and is typically assigned individually in accordance withthe needs of the communications system.

[0033] Each framer of a communications unit is interconnected with theother framer of the communications unit by a short distancebi-directional serial bus 32. An adaptive mode circuit (“AMC”) 34 iscoupled to the two framers and serial bus, and provides the necessaryconfiguration data to the framers to allow them to operate as part of anintegrated, multi-station communications system. The framers alsointerface with the appropriate data sources and processors, such asaudio coders/decoders, under microprocessor control as known in the art,to allow the transmission and reception/processing of digital datathrough the busses 18, as depicted in FIG. 4 and as will be discussedinfra. As each communications unit has two framers, designated as end 1and end 2, communications is enabled in a point-to-point manner in eachof two directions (e.g. “front” and “back”), between a framer and withthe corresponding framer of the communications unit to which it isconnected.

[0034] Under E1/T1 protocols, a framer may operate either in a master,slave or free-run mode. A framer operating in the master mode must beprovided with clock and timing inputs from another source, which is usedto synchronize its output data. A framer operating in a slave moderecovers clock and timing information from the encoded data it receivesfrom another framer over a transmission line. A slave framer can passsuch information to another local framer to which it is coupled.Configuring the coupled framer to operate as a master fulfills themasters need for such information. A framer operating in a free-run modegenerates its own clock and timing information and, like a slave unit,can provide the information to another framer. In this case, however,the framer utilizes an internal local oscillator for the creation of theinformation. Because the clock and timing signals throughout acommunications system must be consistent there can be only one source(i.e., free run framer) of such signals for the system.

[0035] In consideration of the forgoing, the present invention providesfor the designation of the two end or terminal units of thecommunications system (10 or 16 in FIG. 2) to include a free-run framerfor the entire communications system. In communications unit 10 the freerun unit would be framer 24; in communications unit 16 the free run unitwould be framer 26″. Because only one free run framer can be enabled fora communications system, the present invention further provides for theautomated assignment of free run status, and the continued monitoring ofthe communications system to reassign such status as required. In FIG. 3communications unit 12 is shown as including framer 24′, associated withthe unit's “end 2,” serving as a slave, receiving timing and clock dataover the send/receive bus 18 from master framer 26 in communicationsunit 10 and providing it over communications unit 12's internalbi-directional data bus 32 to framer 26′ for “end 1,” framer 26′ servingas a master for a following communications unit (14 in FIG. 2). Thecommunications unit 12 is then fully synchronized to the data which itreceives and sends over the bus channel 18.

[0036] In FIG. 3 communications unit 10 is also depicted with its “end2” framer 24 running in a free-run mode, utilizing oscillator 36 forgeneration of the timing and clock signals. Second framer 26 for “end 1”is configured as a master, and receives its clock and timing signalsover the internal bus 32 from the framer 24, and subsequently passessuch signals to slave framer 24′ in the next communications unit 12.

[0037] As depicted in FIGS. 2 and 3, the present invention designatesone framer of the two framers of an end or terminal communications unitto operate in the free-run mode to generate the timing and clocksignals, and the other framer of the communications unit to operate as amaster to pass the signals to the communications unit to which theterminal or end unit is coupled. The communications unit to which theterminal unit is coupled in turn must have a framer running in a slavemode connected to the end or terminal unit to recover the data and clocksignals from the data transmitted to it from the end unit. The framer atthe opposite end of the connected communications unit must again be amaster to allow it to pass the timing and clock data which it receivesfrom its paired slave unit to the next communications unit andspecifically the slave framer therein. Such a structure is repeated andextends the length of the system.

[0038] Conventional E1/T1 systems do not afford self-configuration; whenused in a point-to-point communications system a framer must beindividually configured by a technician. The present invention allows acontinuous system of communications nodes to be established, andprovides for self configuration of all the framers therein, recognizingthe end or terminal communications units and the individual framerstherein, assigning free run status to one of the available framers ofthe terminal units, and configuring the remaining communications unitsand framers as required.

[0039] With reference again to FIG. 3, it can be seen that the status ofeach framer of each communications unit is controlled by a pair ofinputs, 38, 40 for the first framer, and 42, 44 for the second framer.The two inputs for each framer are designated “master/not free run” and“master/not slave,” respectively. FIG. 3A is a chart depicting thepossible combinations for the two inputs and the resulting configurationfor the framer. As shown, a logic “0” at a first input (38 or 42)configures the framer in free run, irrespective of the value of thesecond input (40 or 44) for the framer. With a logical “1” input at thefirst input, the framer is configured either as a master or a slave,dependent on the value of the second input 40/44; if the second input is“0”, the framer is a slave; if it is a “1” the framer is a master. Inconventional E1/T1 communications systems these values are determined inaccordance with initial system requirements and are hard-wired orjumpered. The present invention generates the control signal pairs foreach of the communications units in the system in a continuing, dynamicmanner to immediately and continuously configure the communicationsunits and their framers for proper operation as communications units areadded and removed to the communications system formed by the individualcommunications units.

[0040] The inputs 38-44 for the two framers of each communications unitare generated by the unit's AMC as depicted in FIG. 5, and moreparticularly the state controller portion 46 thereof depicted in FIG. 7.Each communications unit is provided with an AMC, which both generatescertain data for transmission out by the framers of the communicationsunit and processes both similar data received by the communicationsunit's framers from the communications units to which it is coupled aswell as data generated by the communications unit itself.

[0041] With reference to FIGS. 7 and 7A, the state controller 46portions of the AMC comprises four demultiplexers 48A-D, each of whichreceives three bits of data and utilizes them to generate the fourcontrol bit logic signals to be inputted to the two framers of itscommunications unit. The three data bits are led into inputs S0, S1 andS2 of each of the demultiplexers, forming a 3 bit long binary input wordreferencing one of addresses 0 through 7 (000 through 111) of each ofthe demultiplexers, as presented in FIG. 7A. Each demultiplexer outputseither a high (Vcc) or low (ground) through its respective output O foreach address as set by the data bits or inputs 50-52 in accordance withthe programmed level (high or low) of the associated address D0 throughD7.

[0042] With respect to demultiplexer 48A, for example, addresses D0through D4 are at Vcc, while addresses D5 through D7 are at ground.Thus, when the inputs S0-S2 for demultiplexer 48A form the binary inputword 101 (or 5), the value of D5 (ground or 0) is outputted at 0. Thefour output lines of the four demultiplexers provide the needed framerstate control inputs 38-44 as seen in FIG. 3. FIG. 7A depicts the stateof the two framers (end 1, end 2) corresponding to each address inputtedto the demultiplexers. Framer states for addresses 6 and 7 are notlisted; they correspond to an irrelevant state in a connectedcommunications system.

[0043] The inputs for S0, S1 and S2 of each of the demultiplexers are areceived identifier bit (S0), plus two bits, (S1 and S2), which areindicative of the position of the communication unit in thecommunication system chain. In a preferred embodiment, the positionvalues S1, S2 are derived from internal signals generated by the framersof the communications unit. In particular, each framer in an E1/T1communications unit outputs a signal which is known as LOS (loss ofsynchronization) and which provides information reflecting whether theframer is connected to another framer through its main communicationsbus. This signal is a high/low value, it being high to indicate syncloss and lack of connection. The absence of LOS signals (i.e. thepresence of connections) from both framers of a given communicationsunit is recognized by the AMC of the unit as indicating that the unit isan intermediate unit in the communications system, coupled to othercommunications units at both ends 1 and 2. The presence of a singe LOSsignal, on the other hand, indicates that the communications unit is aterminal or end unit, the particular framer generating the LOS signalindicating the particular uncoupled end of the unit. Thus the AMC, byconsidering the LOS values, can recognize whether the communicationsunit in which it is employed is a terminal unit, capable of beingconfigured with a free-run framer.

[0044] Once the determination is made whether a communications unit isan intermediate unit or an end or terminal unit, one of the two endunits must be configured to include a free run framer for thecommunications system, and the other end unit configured without a freerun framer. The present invention allows the two terminal units to“compete” with each other for free-run framer assignment. Thiscompetition is also conducted and resolved by the AMCs through a random“coin-toss” procedure, utilizing the consideration of identifier bitsgenerated by each of the end or terminal communications units andtransmitted to the other.

[0045] With reference to FIG. 6, upon system startup each AMC enters await state 50, during which the communications unit and its framers areallowed to pass through initial self test procedures, become stable, andgenerate LOS signals. Towards this end the invention takes advantage ofa further characteristic of E1/T1 framers which allows the passage ofdata between framers, although imperfectly, without the framers beingproperly configured, and thus also allows the generation of LOS signals.The presence or lack of presence of the LOS signals is then monitored.Several frames of data containing LOS information may be analyzed toavoid a premature state determination. If the AMC detects the presenceof two LOS signals, the AMC recognizes that the communications unit isan intermediate unit, not capable of participating in the coin tossprocedure. If only a single LOS signal is present, indicating that thecommunications unit is an end or terminal unit, an id exchange state 64is entered by the AMC. (The presence of two LOS signals, represented byaddresses 6 and 7 in FIG. 7A, corresponds to a unconnected unit and thusan invalid configuration for a coupled communications unit.)

[0046] When the id exchange state is entered, the AMC of thecommunications unit first generates an identifier. The identifier ispreferably a single bit “1” or “0,” generated in a random fashion. Asseen in FIG. 5, a free-running oscillator as known in the art (notshown) may be utilized to provide a first clock signal 52, whose outputis latched to that of a second free-running oscillator (also not shown)operating at a different frequency and providing second clock signal 54.The frequencies are arbitrary, but are preferably chosen to be higherthan the E1/T1 frame rate, such that the resulting beat is asynchronouswith the frame rate. Random bit generator 56 uses the two clock signalsto generate a one bit “0” or “1” identifier, which is latched into aregister within the random bit generator. The lack of synchronizationbetween the clock units, coupled with chance, insures that theidentifier latched in the register will be essentially random, and thatthe identifier generated by the other AMC of the other terminal or endcommunication unit, similarly generated, will be different on average 50percent of the time. The identifier bit is placed on line 58 fortransmission out of the connected framer of the communication unit, aswell as being passed to comparator 62. Because data transfer is enabledupon startup, the identifier bit is passed along the entire string ofcoupled communication units until it is received as an “Rx id” bit (seeFIG. 46) by the other terminal or end unit. Simultaneously, the otherterminal or end unit is developing its own identifier and istransmitting it out through its coupled framer to be received by thefirst end unit as its Rx id.

[0047] As the identifier is transmitted, the AMC receives the identifierbroadcast by the other terminal or end communications unit on line 60and enters the verify state 70 in FIG. 6. If the communications unit isan intermediate unit (determined by consideration of its LOS signals)the verify state is immediately entered without generation of anidentifier. The intermediate unit merely passes the identifiers itreceives from the units to which it is coupled from the receiving endand framer to the other.

[0048] In the verify state 70 the AMC of a terminal unit compares thereceived identifier in comparator 62 to the value of its own(transmitted) identifier. For initial configuration purposes, the outputof the comparator, indicating whether the two identifiers are the sameor different, is monitored for 16 data frames at 66, the output of thetest being passed to state controller 46 as an enable signal. If the twoidentifiers are different for 16 consecutive frames the state controlleris so advised and enabled, allowing the state controller to configurethe framers, and the run state 72 (FIG. 6) is entered; thecommunications units being configured for operation.

[0049] If the two identifiers are the same, the AMC in each of theterminal units returns to the wait state at 74 and causes the assignmentof a new identifier for its unit by the random bit generator 56. Thenewly generated identifier bit is again passed to the other terminalunit, and the comparison is again made. New pairs of identifier bits arecontinued to be generated by the terminal communications units andcompared by both units until the 16 frame difference test is met. Eachterminal unit then has a unique identifier assigned.

[0050] Once the difference test is passed and the run state 72 entered,the state controller processor of FIG. 7 in each communications unitprocesses the LOS and identifier data (if needed) to configure theframers of the unit for proper operation. With reference to FIGS. 3A and7A, an Rx id value of 1 configures the respective communications unitwith a free-run framer coupled to a master. The particular framer in thecommunications unit which is configured for free-run is determined bywhich LOS signal is present. As each of the intermediate communicationsunits has no LOS signals, the programmed logic allows Rx id values to beignored.

[0051] With the communications system in the run state 72, The AMC ofeach communications unit continues to monitor the identifiers latchedand passing along the system. When an AMC of a terminal unit encountersthe same received identifier as its own for a minimum of four frames ina series of 16 (block 68 in FIG. 5) the state controller 46 places thecommunications unit back into the wait state 50, causing new identifierbits to be generated and a new assignment of framer configurations to begenerated. While a four frame change can be the result of transientinterruptions in data flow, it also results when a lack of communicationalong the connected units occurs, such as when a rail car is uncoupledfrom the train, or a new communications unit (rail car) is added to anexisting communications system. In either case, the AMCs sense thechange and cause an immediate reconfiguration to occur, re-identifyingeach communications unit in the system as an intermediate or terminalunit, and generating identifiers as appropriate to assign a framer freerun status and reconfigure the entire communications system as needed.The entire reconfiguration typically requires between 20 to 200 framesof data. With typical E1/T1 clock speeds, configuration will take from afew milliseconds to a few tenths of a second. The specific choice offour frames is arbitrary. The number of frames required, however, shouldbe chosen with consideration of the size of the system and the likelyerror rate.

[0052] With proper framer configuration enabled, data is transferredfrom communications unit to communications unit in the system through aseries of point-to-point links established between the coupled framersof adjacent communications units. As E1/T1 framers have a plurality ofdigital data communications channel busses, a plurality of individualchannels can be utilized for a plurality of different types of data. Ina rail car train, for example, intercom and public address signals canbe placed on different channels, in which the audio information isconverted to and transmitted in a digitized form as known in the art,while the transmission of identifier bits is made over another channel.Each communications unit may also include the necessary circuitry, asknown in the art, to digitize generated audio signals and convertreceived digital data to an analog signal as required. The presentinvention utilizes a first set of channels to control the passage of thecommunications data by the use of preamble bits in association with anindividual communication unit's identification data. The preamble data,which may be placed for example on channels 0-3, directs and controlsthe corresponding channels of communications data, for example channels4-7

[0053]FIG. 4 is a block diagram of the interconnection between theframer 24′ and a framer 26′ in a given communications unit, such as unit12 or 14 in FIG. 2. The local communications serial bus 32 between theframer units is shown as a pair of interconnections, one of whichtransmits information from framer 24′ to framer 26′, the othertransmitting data from framer 26′ to framer 24′. In practice, however,each of the lines comprise a plurality of lines or channels, for bothpreamble data and different communications data. Each of the channels(both transmit and receive for each of the framers) is provided with aninsertion multiplexer 76 which allows locally generated data to beplaced on an appropriate channel line and passed to each of the framersfor transmission to the coupled communication units in the system. Eachof the insertion multiplexers 76 are coupled to insert control 78 whichboth directs the delivered data to the proper channel and controls theinsertion timing thereof. Insert control 78 in turn is fed by a seriesof data conversion/processing units, such as codecs 80, each of whichencodes or processes data, such as analog audio data, received from anappropriate input device, such as a public address microphone system,intercom, or the like, and converts it into an appropriate format, suchas a digital format, for ultimate transmission by the framers 24′, 26′.The actual number of codecs or other conversion/processing units will bedetermined by both the number of audio channels required forcommunication purposes and the ultimate capabilities of the framers. Inaddition to receiving the encoded audio data, insert control 78 receivespreamble data on control channels 0-3. The preamble data configures eachof the communications units of the system in a dynamic manner fortransmission and receipt of the audio data on channels 4-7.

[0054] Bus interface 82, which is coupled to a CPU for thecommunications unit, provides the preamble signals for insert controller78 and generates such information to identify the inserted data suchthat it can be passed to selective communication units, as and ifrequired. By placing the preamble information on separate channels fromthe data which it controls, processing of the preamble information canbe performed without risk of corrupting or dropping portions of thecontrolled data.

[0055] In addition to the serial bus 32 being provided with insertionmultiplexers 76, it is also provided with a pair of demultiplexers 84which allow the extraction of data received by the framers. Thedemultiplexers extract the received digital data on each of the channelsand, through control switches 86, pass the data for appropriateprocessing, including decoding into analog signals as appropriate. Amongthe output of the demultiplexers 84 is received preamble identificationdata of the type added by insert control 78. The received preambleinformation is detected by preamble detector 88 which in turn is coupledto the switches 86, controlling the switches to allow the receivedcommunications data to be decoded if received preamble informationindicates that it is destined for the particular communications unit.The output of the preamble detectors is also led to timing controller90, which provides the selective timing for the insertion of data toinsure that generated data for transmission out does not conflict orcollide with data generated by other communications units passing alongthe system.

[0056] As shown in the figure, both sets of transmit/receive channels ofserial bus 32 are provided with both insert and multiplexers 76 anddemultiplexers 84. In operation, however, communications across theserial bus are enabled only in one direction. That is, only onemultiplexer and demultiplexer of a communications unit is active, eachactive device being associated with a different group of channels. Oneof the end or terminal communications units is configured in a loop-backmode, such that the received data is re-transmitted back to thecommunications unit to which the terminal unit is connected. Thisensures that, even though transmissions between communications unitsflow in only direction, all transmissions travel throughout the system.The direction in which the data passes is not critical. Indeed, thechoice of direction may be made in a random manner. Because thecoin-toss procedure discussed above provides for dynamic reallocation offree-run status to a terminal unit, it may be efficient to use the sameindicator bit to control data flow direction. Use of such a bit alsoallows for dynamic reconfiguration of the communications units at thecommunications level as units are added or dropped to the system.

[0057] Each communication units CPU is aware of the nature of thecommunications unit as either a terminal/end unit or an intermediateunit, and also whether, if a terminal unit, the unit is set for freerun. In a preferred embodiment of the invention, the communications unitset for free run is also designated the parent unit for communicationspurposes. Accordingly, that unit controls the communication traffic onthe system. This is done by requiring a communications unit to seekcontrol of a communications channel before data is placed thereon. Byhaving one communications unit, and particularly the designated parentunit supervise all communications, conflicting and colliding data to oneor more child units on channels are prevented.

[0058] When a child unit requires access to a bus for transmissionpurposes, it must first obtain “permission” from the parent unit.Preamble data on control channel is utilized for such purposes. Allchildren monitor the control channels on one end and retransmit thereceived preamble data out the opposite end from which it is received.

[0059] The parent unit polls each child unit individually on a periodicbasis to determine if a child unit requires access to a bus for atransmission. Again, the polling may be performed by the transmission ofappropriate preamble data. The poll request is passed down the line fromcommunications unit to communications unit, allowing it to reach theunit to which it is directed. As each communications unit has anindividual identification, a poll request can be designated for aparticular unit and can be recognized as such by each unit in the systemas it is passed therealong. When the poll request reaches the polledchild, the child unit transmits a poll response within a specified pollresponse period, which is again passed by one of its framers out theappropriate end of the unit and passed from unit to unit until itreaches the parent unit. After each child has been polled and given apoll response opportunity, the parent issues a “poll response fromparent” preamble message, typically granting authority to transmit on aparticular channel. The poll response from parent message is similarlypassed along the line from communications unit to communications unitand is acted upon as required by each child unit. An “event” can beattached to either a poll request or poll response from parenttransmission. This can be for example, an instruction to each child in arail car system unit to update a sign message board in the carassociated with the child unit. The event is received and re-transmittedby each child along the communications system, insuring that all unitsreceive the event information and act on it accordingly in accordancewith the preamble information associated with the transmission. Again,as each communication unit has a specific identifier associated with it,events can be directed to specific units as required. All of theforegoing is preferably implemented and controlled as known in the artby the communications units' cpus.

I claim:
 1. A multi-point communications system, comprising a pluralityof communication units coupled together in a series point-to-pointconfiguration, each of said communications units comprising: a firstE1/T1 protocol framer having a communications port for point-to-pointcommunication with a first remote communications unit and a localcommunications port; a second E1/T1 protocol framer having acommunications port for point-to-point data communications with a secondremote communications unit and a local communications port; and controlmeans coupled to the first and second framers for controlling thepassage of communications data generated by the communications unit tosaid first and second framers for transmission to the coupled remotecommunications units and for controlling the passage of communicationsdata received by one of said first and second framers from a remotecommunication unit to the other of said first and second framers forfurther transmission to another remote communications unit.
 2. Thecommunications system of claim 1 wherein said control means is coupledto the local communications ports of the first and second framers. 3.The communications system of claim 2 wherein said control means comprisemeans for placing preamble data on a first group of channels andcommunications data controlled by said preamble data on a second groupof channels.
 4. The communications system of claim 3 further comprisingmeans in each of the communications units for dynamically assigning oneof the framers of one of the communications units of the communicationssystem free run status to provide timing signals for the communicationssystem.
 5. The communications system of claim 4 wherein said dynamicassignment means includes means for identifying terminal communicationsof the communications system and assigning free run status to to one ofthe framers of one of the terminal communications units.
 6. Thecommunications system of claim 5 wherein said dynamic assignment meansfurther includes means for assigning parent status to the communicationsunit having the free run status framer.
 7. The communications system ofclaim 6 wherein said communications unit having parent status controlsthe sequence and timing of data communications along the communicationssystem.
 8. The communications system of claim 4 wherein said controlmeans further comprise a pair of multiplexers each coupled to the localcommunications ports of the first and second framers and means forselectively enabling one of said multiplexers to direct the direction ofcommunications between communications units.
 9. The communicationssystem of claim 8 wherein said control means further comprises a pair ordemultiplexers each coupled to the local communications ports of thefirst and second framers.